1. Field of the Invention
The present invention relates to a rotor having projections for positioning permanent magnets mounted on an outer surface thereof, and to an electric motor including such a rotor.
2. Description of the Related Art
In an SPM (Surface Permanent Magnet) type of rotor of an electric motor, in which permanent magnets are mounted on a surface of a sleeve, cogging torque generated due to magnetic attraction and repulsion forces acting between the permanent magnets and stator teeth tends to have a relatively greater influence to the performance. Since cogging torque makes it difficult to accurately control the electric motor, it is necessary to accurately mount the permanent magnets in place in order to prevent cogging torque from unexpectedly generating. A known rotor has a configuration, in which a sleeve is provided with projections on an outer circumferential surface thereof whereby movement of the permanent magnets is restricted to prevent the permanent magnets from dislocating. JP-A-2007-006621 discloses a rotor in which the permanent magnets having a shape which gradually decreases from one end toward the other end are alternately arranged so as to be oriented in opposite directions, in order to position the respective permanent magnets at equal intervals.
There is a need for a rotor which allows the permanent magnets to be accurately positioned in place, and for an electric motor including such a rotor.